A fluid barrier curtain maintains the separation of fluids on opposite sides of the curtain while allowing objects to pass therethrough. Such a curtain is especially useful in industries where the controlled preservation of a fluid treatment zone is necessary. One example of use of fluid barrier curtains is with heat treating furnaces. Typically, a mechanical conveyor is employed to carry a continuous series of articles to be heat treated into the furnace through an access aperture, through the tunnel space in the furnace, and out of the furnace through another access aperture. A requirement of some industrial heat treating furnaces and ovens of the continuous type is maintenance of carefully controlled and dried atmosphere within the heat treatment chamber in the furnace. Optimum control of furnace atmosphere requires maximum exclusion of infiltrating outside air, minimizing inter-diffusion of side-by-side separate atmospheres and minimizing intermixing between the furnace atmosphere and the curtain gas.
Prior to the present invention, it has been the practice to provide physical baffles, such as hinged metal doors, or flame curtains located at the entrance and exit of the heat treating chamber for resisting the influx of atmospheric air. These devices tend to be only marginally satisfactory in eliminating intermixing of gases in heat treating apparatus. Baffles arranged for movement when workpieces are introduced in a continuous furnace system allow intermixing of outside air into the internal atmosphere with consequent non-uniform heat treating results. Hydrogen furnaces, since they are particularly sensitive to mixing of trace air, are sometimes constructed in a so-called "hump back" design in order for the lighter hydrogen to drift to the top of the furnace, thus aiding in excluding outside air but at the expense of flame burning curtains continuously burning off hydrogen at the air-hydrogen interface. Flame curtains require large amounts of expensive hydrogen, create water vapor as a by-product, allow carbon deposits to appear on the workpiece, and otherwise provide additional heat for the workpiece that may be objectionable.
Heat treating results using state of the art gas curtains allow intermixing of curtain gas and interior furnace gas resulting in variable reducing conditions.
One form of gas curtain for industrial furnaces is described by Francis, et al. in U.S. Pat. No. 4,448,610 issued May 15, 1984. Inert gas emerges from holes in a tube located at the ceiling of a furnace opening, the holes being angled inward in an attempt to force back furnace atmosphere. A similar system is described by Colby in U.S. Pat. No. 1,725,059 except that a small tube with holes directs curtain gas straight downward across a furnace opening. Between the times of these patents, other patents show varying gas curtain schemes of similar overall approach. For example, forced blown gas angled inward toward furnace atmosphere, intended to be recirculated and re-exhausted, is disclosed by Nowack in U.S. Pat. No. 4,298,341 issued Nov. 3, 1981. A perforated pipe gas curtain arrangement for furnace atmosphere control is described in U.S. Pat. No. 3,931,684 by Trumball, et al. In this disclosure humid air issues from holes in a pipe toward a narrowed inlet heating chamber where baffles together with an exhaust pipe are intended to reduce escape of volatile gases from the curing chamber.
Foeling et al. describe in U.S. Pat. No. 3,672,948 a transverse flow of gas across wafers transported longitudinally along a furnace with little concern for intermixing of ambient air except by narrowing of the input port. Tamm's U.S. Pat. No. 3,363,533 shows a blower operated nozzle blowing forced air at an angle between 15 degrees and 25 degrees so as to split the airflow into two unequal horizontal currents--a major current directed toward the warm interior atmosphere of the furnace, and the second current directed toward the colder atmosphere at the end of the furnace whereby this current stream rebounds from the furnace floor.
An apparatus showing a spray discharge downward against the flow of a furnace opening combined with an exhaust chimney arrangement is disclosed by Thompson in U.S. Pat. No. 3,223,396.
Gas curtain apparatus for protecting an area for surgical operation is disclosed in U.S. Pat. No. 4,140,015 by Duvlis wherein sterile air is ejected from a blower, directed across a surgical operating area, and then aspirated by a respective intake of a suction unit. A sterile curtain of air directed across an opening during the time a door is opened in an enclosure is described by Copp in U.S. Pat. No. 3,221,632.
Air curtains for doorways are described by Jansson in U.S. Pat. No. 4,074,620 wherein air jets forth from slots in a doorway so as to attempt prevention of cold air flowing into a room. Another doorway air curtain is described by London in U.S. Pat. No. 3,086,441 wherein air is forced downward as by a blower discharging it from nozzles across a doorway. Zehnder describes in U.S. Pat. No. 3,068,775 varied vane airscreens directing forced air toward cold air to keep it from entering a room.